1. Field of the Invention
The invention relates to a DCxe2x80x94DC converter, a regulation method for a DCxe2x80x94DC converter, and a switched-mode power supply.
2. Description of the Related Art
DCxe2x80x94DC converters, also denoted converters, are known for converting an input-side DC voltage into an output-side DC voltage. A DC voltage present on the input side is first converted (chopped) into a switched AC voltage, and this AC voltage feeds a circuit comprising the primary side of a transformer. In this way, at least one voltage, which, after rectification, is available as an output DC voltage, is generated on the secondary side of the transformer.
Such DCxe2x80x94DC converters are also known as a decisive assembly of switched-mode power supplies. They comprise a switched-mode power supply input circuit for connection to the electricity power grid and for generating an intermediate circuit DC voltage. The intermediate circuit DC voltage feeds the incorporated DCxe2x80x94DC converter.
Numerous circuits for DCxe2x80x94DC converters are known. They comprise, on the one hand, very simple circuits in which the inverter only consists of a single controlled switch. In these simple circuits, the switched AC voltage is directly fed to the primary side of the transformer, so that the primary-side circuit thus consists of only the primary side of the transformer. On the other hand, also resonant converters are known in which a primary-side circuit is supplied with a switched-mode AC voltage generated by a half-wave bridge or a full-wave bridge, the primary-side circuit being built up from at least one capacitance and at least one inductance, the inductance often not being present as a discrete component, but as the stray inductance of the transformer.
European Patent Application No. EP 0 898 360 discloses a method and a device for controlling a DC rectifier with an AC intermediate circuit. An inverter with a controlled half-wave bridge or full-wave bridge generates an AC voltage which is transformed via a transformer and rectified on the output side. The secondary side of the transformer then comprises a winding with a middle tap, while the output voltage is rectified by two-way rectification. In this publication, the problem is discussed of the saturation of the magnetic flux in the transformer core. To achieve a waveform here which shows the least possible saturation it is proposed that the switched-mode AC voltage generated by the inverter is symmetrical, i.e., that within a predefined time interval, a voltage pulse is generated that is as long positive as it is negative. This publication, however, exclusively discusses a control method and not a regulation of the output voltage. In this case, a converter topology is provided which does not utilize a resonant arrangement.
In resonant converters, it is a known fact that the output voltage is regulated, the drive of the inverter forming the setting variable then. If, for example, a half-wave bridge generates a pulse-width modulated voltage for supplying power to a resonant arrangement mentioned above, the output voltage can be regulated by varying the frequency of the pulse-width modulated voltage, i.e., the closer the switching frequency is to the resonant frequency, the more noticeable is a resonant voltage increase and the higher the (rectified) output voltage is.
U.S. Pat. No. 5,986,895 describes a DC current converter comprising an inverter driven by a pulse-width modulation for supplying power to a resonant circuit in the form of a resonant capacitance and the primary side of a transformer. The secondary-side voltage of the transformer is rectified and filtered at a secondary winding having a middle tap thereby forming two branches each including one diode and each generating a DC output voltage. The output voltage is regulated via the drive of the inverter with variable pulse width. To reduce losses during switching, the current through the resonant circuit is measured and the switching times are selected such that, when a minimum and a maximum frequency are adhered to, switching only takes place when the current reaches a lower threshold value. U.S. Pat. No. 5,986,895 additionally describes the problem of losses during the rectification of asymmetrical output voltages. This problem is solved by the use of secondary-side synchronous rectifiers instead of diodes.
As a matter of fact, secondary-side asymmetries in the paths containing rectifier elements (diodes) form a problem for DCxe2x80x94DC converters. secondary-side asymmetries arise in that, with multipath rectification (for example, by a four-diode bridge or by two single diodes with a winding with a middle tap), the individual branches, and thus the individual rectifier elements, are loaded differently, i.e., different powers, voltages or currents are to be managed. This leads to the fact that when the circuit is designed, the asymmetrically loaded components are to be designed according to their respective maximum load. For example, components are to be dimensioned with a larger, for example, double power dissipation, so that, partially, additional cooling measures are necessary. Also, any output filter for filtering the rectified output voltage needs to be able to process the lower frequency portions (sub-harmonics) resulting from asymmetry and to have a larger structure for this and also for a higher current standing wave ratio.
A reason for such asymmetries may be, for example, tolerances in the stray inductance of the output-side windings or tolerances in the forward voltages of the diodes. Building the circuits for avoiding such asymmetries with the aid of components that have smaller tolerances leads to highly increased cost. Furthermore, the tolerances cannot always be avoided because, for example, the forward voltage of a diode depends on temperature.
Accordingly, it is an object of the invention to provide a DCxe2x80x94DC converter and a regulation method for a DCxe2x80x94DC converter in which asymmetrical loads of the secondary-side rectifier are avoided to a maximum extent.
This object is achieved by a DCxe2x80x94DC converter comprising an inverter for generating a switched AC voltage, a primary-side circuit supplied with power from the inverter, the primary-side circuit comprising the primary side of a transformer, the transformer having a secondary side including at least one rectifier for generating at least an output DC voltage (Vo), a measuring unit for measuring an electrical magnitude of DCxe2x80x94DC converter, a symmetry calculation unit for calculating, from the measurement, a parameter for the symmetry deviation of the electrical magnitude, and a symmetry regulation unit for changing the drive of the inverter in dependence on the symmetry deviation so that the symmetry deviation is minimized.
According to the invention, asymmetrical loads are avoided in that an electrical magnitude is measured and a symmetry criterion is applied thereto. Accordingly, driving the inverter then provides that it is excited so that the output load is symmetrical.
In the DCxe2x80x94DC converter according to the invention, a measuring arrangement is provided for measuring an electrical magnitude where various primary and secondary-side magnitudes are involved. For example, the primary-side current can be measured by the transformer. Likewise, it is possible to measure the voltage at a primary-side capacitance. A further possibility comprises the measuring of a secondary-side magnitude, preferably the rectified secondary-side voltage. Accordingly, suitable measuring arrangements are sufficiently known to the person of ordinary skill in the art.
A symmetry calculator unit uses the measuring result to calculate therefrom a parameter for the symmetry deviation from the electrical magnitude. Preferably, the waveform of the electrical magnitude is considered here, and for an observation interval, the symmetry thereof is determined, i.e., the deviation from a symmetrical waveform. Various criterions may be used for this, according to which the symmetry is judged and a parameter for the deviation of symmetry is formed.
According to a first further embodiment of the invention, a parameter for the symmetry deviation is formed in which the substantially periodic waveform of the electrical magnitude under consideration, preferably the voltage on a primary-side capacitance, is expressed in a time interval with at least a local minimum and a local maximum of the waveform. A parameter for the symmetry deviation here forms a magnitude that depends on the difference between the values of the respective peak values. xe2x80x9cDependsxe2x80x9d is here to be understood to mean that the difference between the values can directly express the value for the symmetry deviation, but it is also possible for further mathematical operations to be applied to this value, for example, multiplication by a constant factor.
A further proposal is that a magnitude is given as a parameter for the symmetry deviation, this magnitude being calculated from the different-sized deviations of a maximum and a minimum value from a mean value of the electrical magnitude. The respective deviations (differences) are determined and compared with each other (difference of the values). The parameter for the symmetry deviation is calculated from this difference while, here too, further mathematical operations can be applied.
According to a further proposal, a criterion is used as a parameter for the symmetry deviation, according to this criterion, the waveform of the electrical magnitude is considered, and within a first time interval, an extreme, i.e., a minimum or maximum value is determined. Likewise, an extreme value is determined for a second time interval. A parameter for the symmetry deviation can be calculated from the difference between the extreme values. This parameter is particularly suitable for considering a rectified magnitude, for example, of the secondary-side rectified voltage. Maximum values in successive time intervals, preferably in the excitation intervals of the pulse width modulated switched AC voltage, are a parameter for the power transferred via the respective branch of the rectifier. A deviation of the extreme values of the two time intervals considered is thus a parameter for asymmetrical load.
It is generally possible for the symmetry calculator unit to first calculate an intermediate magnitude from the measured magnitude. For example, by integrating the current at a capacitance, an intermediate value, whose waveform corresponds to that of the voltage at the capacitance, can be determined at a capacitance over time. The criterions indicated for determining the symmetry deviation, can then be used for this intermediate magnitude.
In addition, more criterions may be found which are suitable as a parameter for the symmetry deviation. A person of ordinary skill in the art will choose the most favorable criterion as regards cost and requirements for a particular application.
The symmetry deviation determined thus is applied, according to the invention, to a symmetry regulation device which predefines the drive of the inverter so that the symmetry deviation is regulated to zero. It has turned out that an even load of the output rectifiers is achieved, i.e., better or worse depending on the criterion used, by regulating the electrical magnitude in a way that a symmetrical waveform is achieved relative to the respective criterion used. Even with components that have relatively large tolerances, it is possible to attain an even load of the output rectifier.
In a preferred embodiment of the invention, the inverter is driven so that it produces a pulse-width modulated AC voltage. This is understood to mean an AC voltage which is featured by a switching frequency and a duty cycle, while within each time interval whose duration is determined by the switching frequency, first a positive and then a negative voltage pulse is generated (full-wave bridge) or first a positive voltage pulse and then a zero output voltage is generated for a period of time (half-wave bridge). The duration of the positive voltage pulse is predefined by the duty cycle (ratio of the duration of the positive voltage pulse to the total time of the interval) and the duration for which the voltage is zero or negative is predefined by the rest duration of the interval. The symmetry regulation device here preferably uses the duty cycle as a setting quantity to regulate the symmetry deviation to zero, i.e., to achieve a symmetrical waveform of the electrical magnitude relative to the respective symmetry parameter.
A symmetry regulation device that respectively predefines the duty cycle can be combined in a simple manner with a conventional known regulation device for regulating the output voltage. The output regulation device can then regulate, for example, the output voltage and predefine the frequency of the pulse width modulated AC voltage in known manner, whereas, the symmetry regulation device predefines the duty cycle. In this way, a DCxe2x80x94DC converter can be arranged for producing a regulated DC output voltage while the power semiconductors (rectifiers), the transformer and the output filters, as required, can be utilized optimally.
A symmetry regulation device and a symmetry calculator unit are understood to mean purely functional units, i.e., in an actual device, such units need not of necessity be separate assemblies. A symmetry calculator unit as well as a symmetry regulation device can be realized as an analog circuit. A person of ordinary skill in the art is familiar with respective circuits for making mathematical calculations (for example, difference formation, multiplication by a constant, averaging, integration, etc.). Also analog circuits for realizing a regulation device are known, such as a simple proportional regulator or a PID regulator. Alternatively, the symmetry calculator unit and/or the symmetry regulation device may also be realized as a discrete or integrated digital circuit, or completely or in part designed as a program running on a microprocessor or signal processor.
In an actual realization, it is also possible to realize, in one assembly, both the symmetry regulation device according to the invention and a known output regulation device for the regulation of the output voltage.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.